The present invention relates to output buffers for translating from circuitry operating at one voltage level to another voltage level, and in particular from 1.9V to 3.3V.
Transistor circuitry for logic chips will typically operate between a supply voltage and ground. One common supply voltage in the past has been 5V. More recently, circuit devices operating with a 3.3V supply have been developed, with other circuits being designed for 2.5V supplies and 1.9V supplies. Future circuits may use even lower voltage supplies.
The need for lower voltage supplies results from a number of factors. A lower voltage typically means that less power is consumed by the circuit, which is especially important in battery-operated devices. In addition, a lower voltage typically means a shorter transition time for signals, and thus faster clock speeds since the signal only needs to rise up to a lower voltage level.
A number of obvious difficulties arise in designing circuits to operate under lower voltage supplies. In particular, transistors require a certain amount of voltage to exceed their threshold and to turn on and off. As the voltage supply is lowered, the susceptibility to noise increases, since the same noise signal is a larger percentage of the data signal.
Additionally, there is a need to interface between circuits of different voltage levels. In particular, certain devices may standardize on one voltage level, while other devices may standardize on another voltage level, and they need to talk to each other. For example, a 3.3V bus has been adopted for certain computer systems. This is often adequate for the bus and certain peripheral logic chips or memory attached to the bus. However, in microprocessors, there may be a desire for lower voltage levels. In order to increase the speed in a microprocessor and reduce its power consumption while more gates are placed on it, a lower voltage supply is needed.
Buffer circuits have been designed to interface between circuitry of two different voltage levels. In interfacing between 1.9V and 3.3V, for instance, a particular difficulty arises when the transistors in 1.9V fabrication process can withstand only a voltage of 1.9V. Greater voltage levels could damage the transistors. Accordingly, the problem of driving a 3.3V output with circuitry that cannot put more than 1.9V across a transistor poses unique challenges. One approach for doing this is a cascode transistor arrangement in which the 3.3V is essentially applied across two transistors in series, with one being attached to a voltage reference to prevent too much voltage being applied to the other switching transistor. Typically, these two transistors should be connected between 3.3V and the output pad, and another two transistors connected between the output pad and ground or the low voltage reference.